1. Field of the Invention
The present invention relates to a receiver apparatus comprising display means for displaying signal strengths of signals having a plurality of reception frequencies, and a display apparatus for use in a receiver apparatus, such as a radio receiver, a radio transceiver, a radio FM transceiver or the like.
2. Description of the Related Art
FIG. 2 shows a conventional radio receiver having a spectrum display function. Referring to FIG. 2, a high-frequency signal received by an antenna 100 is input through a high-frequency amplifier (referred to as an "RF amplifier" hereinafter) 101 to a mixer 102, and then, is mixed with a local oscillation signal so as to be converted into an intermediate frequency signal (referred to as an "IF signal" hereinafter). The IF signal is input through an intermediate-frequency amplifier (referred to as an "IF amplifier" hereinafter) 104 to an FM demodulator 105, which frequency-demodulates the input IF signal and then outputs the resulting low-frequency signal or voice signal. Thereafter, the resulting low-frequency signal obtained through the FM demodulation is output through a low-frequency amplifier (referred to as an "AF amplifier" hereinafter) 106 to a speaker 107. The IF signal output from the IF amplifier 104 is subjected to envelope detection in a detector circuit 108, and then, the detected signal is output to a signal level meter 109, so that the signal level of the received high-frequency signal is displayed on the signal level meter 109.
Meanwhile, the IF signal is input to a mixer 110 and then mixed with a sweep local oscillation signal generated with a predetermined sweep frequency width by a sweep local oscillator 111, and then, the IF signal is converted into a sweep IF signal having the above-mentioned sweep frequency width. The sweep IF signal is amplified by a scope IF amplifier 112, and then, the amplified sweep IF signal is subjected to envelope detection in a detector circuit 113, which outputs the resulting detection signal. The resulting detection signal is input to a vertical axis input terminal of a CRT display 114. The sweep local oscillation signal output from the sweep local oscillator 111 is further input to a horizontal sweep signal generator 115 including a PLL (Phase Locked Loop) circuit. The horizontal sweep signal generator 115 generates a horizontal sweep signal for the CRT display 114 in synchronous with the input sweep local oscillation signal, and then outputs the generated signal to a horizontal axis input terminal of the CRT display 114. With the above-mentioned arrangement, the frequency spectrum of the signal received by the antenna 100 is displayed on the CRT display 114, wherein the frequency spectrum has the predetermined sweep frequency width around the center frequency of the received high-frequency signal.
Therefore, the circuits and apparatuses denoted by the reference numerals 110 through 115 constitute so-called a spectrum analyzer for displaying a frequency spectrum, which allows the operator to observe the frequency spectrum including not only the high-frequency signal having the frequency but also signals having frequencies adjacent thereto, while hearing the demodulated low-frequency signal contained in the received high-frequency signal, which is output from the speaker 107.
However, the conventional radio receiver having a spectrum display function is provided with two receiver circuit systems for receiving a first reception frequency and a second reception frequency: one system composed of original signal receiver circuits extending from the RF amplifier 101 to the IF amplifier 104 which are provided for receiving a high-frequency signal at the first reception frequency; and another system composed of the receiver circuits for spectrum display extending from the RF amplifier 101 to the scope IF amplifier 112 including the other local oscillator 111, which are provided for receiving a high-frequency signal at the second reception frequency. The above-mentioned fact has been resulted in the problems of a complicated circuit and an expensive manufacturing cost.
Furthermore, since the local oscillator 111 and the local oscillator 115 are interlocked in operation in the conventional radio receiver having a spectrum display function, the signal strengths of frequencies continuously adjacent to the output signal of the mixer 102 can be observed. However, it has been impossible to observe the signal strengths of signals having a plurality of discrete frequencies which are previously determined.
Furthermore, the conventional radio receiver having a spectrum display function has such a disadvantageous problems that the signal strength at the frequency obtained prior to changing the frequency is displayed until the signal strength of the reception frequency corresponding to the display position is measured in spite of the fact that the display position of the spectrum obtained after changing the frequency is required to be displayed when the reception frequency is changed.